Utility poles are widely used to support overhead power and telecommunication lines. Wooden utility poles are pressure impregnated before installation with materials such as creosote to minimise rotting but this still occurs, usually from the centre outwards.
The reasons for rotting usually are that
(a) the preservative does not penetrate to the centre of the poles; and
(b) some soils contain chemical compounds that are particularly aggressive even towards treated timbers.
Any rotting puts the poles at risk due to failure at or just above ground level where the maximum bending moment is applied. High bending stresses occur during extreme weather conditions and even new poles can be broken. For this reason poles which have lost more than 40% of their integrity (i.e. have a strength less than 40% of their original nominal strength) are replaced. This is not always easily accomplished as poles are often located in sites inaccessible to transport so that lengthy disruption of services can occur. Even though they may rot, wooden poles are still preferred in many parts of the world because of the availability of the wood (and they are comparatively easily climbed by a properly equipped workman). Alternatives to wooden poles such as reinforced concrete and glass reinforced plastics can also suffer damage at or about ground level.
The present invention is designed to provide a means and method for the in situ repair of utility poles.
Such a repair system to be viable should be capable of reinforcing poles to an acceptable strength equivalent to that of new ones, should be easy to accomplish on site, should need access only to the base of the pole so that there is no disruption of services, and should be resistant to corrosive and other attack so as to give a pole a long life without further main- tenance.
Various systems for repairing elongate members have been proposed in the art.
For example, GB-A-1489518 shows a way of repairing piles underwater by cutting away a rotten part of the pile, surrounding it with a bag and pouring cement into the bag. The rotten part is effectively replaced by the concrete. The concrete, which may have a larger dimension than the original pile, is the only added load-bearing element. A small excavation may be made into the earth at the bottom of the pile and concrete may enter it, but it is not surrounded by the bag at that position. The purpose is to resist vertical loads.
GB-A-1550403 shows a way of strengthening structural tubes of an oil-rig by surrounding a damaged part by a sleeve, filling it under pressure with a hardenable composition and maintaining the pressure until the composition has hardened.
There have also been proposals for setting poles in their new condition into the earth and protecting them against rot; by filling a cavity in the earth with foam and setting the pole in it (GB-A-1199725); by forming a concrete pot in a cavity and then packing a pole into the pot with rubble or the like which is filled with a preservative (GB-A-429665); by setting them in a sleeve in the ground of which the upper end just projects from the surface (GB-A-433428); or by forming a solid protective layer on the pole before it is inserted into the ground (GB-A-125068).